This invention relates to improvements to processes for the production of panels and other articles from lignocellulosic particles and binder.
There are currently many different types of panels that are produced using wood as the raw material and bonded with one or more types of binders (i.e. glues). These panels include Oriented Strand Board (xe2x80x9cOSBxe2x80x9d), High Density Fibreboard (xe2x80x9cHDFxe2x80x9d), Medium Density Fibreboard (xe2x80x9cMDFxe2x80x9d) and Particleboard (xe2x80x9cPBxe2x80x9d). The principle behind all of these products is to construct a mat using wood particles such as flakes or fibres and some type of glue and then to subject the mat to pressure and heat until the resin cures and the mat becomes a composite product.
The overwhelming majority of binders which are used in wood panel products are thermosetting formaldehyde based resins, which are selected primarily because of their low cost. Examples of such binders include urea formaldehyde resin (xe2x80x9cUFxe2x80x9d), melamine formaldehyde resin (xe2x80x9cMFxe2x80x9d or xe2x80x9cMUFxe2x80x9d) and phenol formaldehyde resin (xe2x80x9cPFxe2x80x9d). Other binders which are sometimes used include isocyanate based glues such as methylene diphenyl isocyanate (xe2x80x9cMDIxe2x80x9d) and its various polymeric forms. Although isocyanate based binders offer potentially a stronger bond between discrete wood particles, they are also very expensive relative to the formaldehyde based resins.
An emerging product is strawboard, which may be similar to OSB, HDF, MDF or PB but is constructed from straw particles instead of wood particles. Strawboard is potentially desirable from a resource conservation perspective because straws such as cereal straw are an annually renewable resource and are a by-product of agricultural operations, while wood supplies are diminishing as demand for wood products increases.
In the past, it has been found that isocyanate based binders such as MDI are preferred as the primary binder in the production of strawboard because strawboard produced with formaldehyde based resins such as UF tend to exhibit a significantly lower internal bond strength than those produced with MDI.
This lower internal bond strength has conventionally been attributed to a wax coating which covers straw particles and which has been believed to interfere with the adhesion between straw particles and relatively weak bonding agents such as UF. Unfortunately, the need to use expensive glues such as MDI in the production of strawboard has conventionally resulted in the strawboard product being costly to produce and thus of only marginal commercial value.
In addition to their expense, there are other problems associated with isocyanate based binders such as MDI. First, MDI is a very reactive substance and will bond with many material types including metals such as those typically used in press platens. This characteristic necessitates the use of release agents or some kind of physical barrier between the platens and the mat to be pressed, both of which result in added expense and potential production problems. Second, MDI vapour is hazardous to health and must therefore be managed accordingly. Third, MDI in its uncured state lacks xe2x80x9ctackxe2x80x9d or stickiness and thus does not impart any structural integrity to the mat before it is pressed. This characteristic means that the mat must be handled very carefully before pressing or it could fall apart. A fourth consideration is that laminate overlays do not bond to an MDI bonded surface as well as they do to a UF bonded surface.
There is therefore a need for a strawboard product which can be produced at least in part from glues other than isocyanate based binders.
The present invention is an improvement on a process for producing articles from lignocellulosic particles, wherein the process comprises the step of combining the lignocellulosic particles with a binder. In particular, the invention comprises the step of treating the particles of lignocellulosic material prior to combining them with the binder by exposing the lignocellulosic particles to an acidic environment.
Although the invention may be used effectively with any lignocellulosic particles including wood particles, the lignocellulosic particles are preferably comprised of straw particles of any type. More preferably the straw particles are selected from the group of plants consisting of barley, wheat, oats, canola, flax, hemp, bagasse, rice, sunflowers, hay and grass. Most preferably the straw particles are selected from the group of plants consisting of barley, wheat, oats and canola.
The step of treating the lignocellulosic particles is preferably comprised of combining the lignocellulosic particles with an aqueous acidic solution, which is preferably a dilute solution of a weak acid such as a carboxylic acid (preferably acetic acid), a very dilute solution of a strong acid such as hydrochloric acid or sulphuric acid, or a combination thereof. Preferably the aqueous acidic solution has a hydrogen ion concentration of between about 0.0025 moles per liter and about 0.006 moles per liter. Preferably, the treating step is comprised of combining the lignocellulosic particles with a sufficient amount of aqueous acidic solution so that the straw particles are combined with between about 0.001 and 0.003 moles of hydrogen ions per kilogram of oven dry straw particles. The straw particles may be combined with the aqueous acidic solution by spraying the straw particles with the aqueous acidic solution.
Preferably, contact between the straw particles and the aqueous acidic solution is then maintained for a period of time. Preferably contact is maintained for between about 5 minutes and about 60 minutes, and more preferably for between about 10 minutes and about 30 minutes.
The lignocellulosic particles may have any moisture content for the treating step, but preferably they have a moisture content of less than about 20 percent by oven dry weight of lignocellulosic particles when they are combined with the aqueous acidic solution. Most preferably, the lignocellulosic particles have a moisture content of between about 5 percent and about 20 percent by oven dry weight of straw particles when they are combined with the aqueous acidic solution.
The lignocellulosic particles and the aqueous acidic solution may be at any temperature for the treating step, but preferably at least a portion of the treating step occurs at a temperature of at least about thirty degrees Celsius.
The invention may also be comprised of the step of combining the straw particles with the binder. The binder may be any binder which is suitable for gluing lignocellulosic particles, including both formaldehyde based binders and isocyanate based binders. Preferably, however, at least a portion of the binder is a formaldehyde based resin such as an urea formaldehyde resin (xe2x80x9cUFxe2x80x9d), a melamine formaldehyde resin (xe2x80x9cMFxe2x80x9d or xe2x80x9cMUFxe2x80x9d) or a phenol formaldehyde resin (xe2x80x9cPFxe2x80x9d). Mixtures of different binders may also be used. For example, a formaldehyde based resin such as UF may be used for some portions of the article while an isocyanate based binder such as MDI may be used for other portions of the article. Preferably the lignocellulosic particles have a moisture content of less than about 15 percent by oven dry weight of straw particles by the time that the step of combining the straw particles with the binder is finished.
The lignocellulosic particles may be of any size, but preferably they have a maximum dimension of less than about 200 millimeters during the treating step. More preferably they have a maximum dimension of less than about 50 millimeters during the treating step. Most preferably, the lignocellulosic particles have a maximum dimension of less than about 25 millimeters during the treating step.
The invention may be used to produce any type of article from the lignocellulosic particles and the binder. Production of the article comprises the step of forming the article from combined lignocellulosic particles and binder followed by the step of curing the binder to produce the article. Preferably the binder is a thermosetting binder which is cured by heating of the article. Preferably the article is a panel which is formed by creating a mat of combined lignocellulosic particles and binder. The binder is then cured to produce the panel.